While trying to understand Sphinx, I had a few troubles and tried to improve
what I found difficult.

In particular:
* Why "addnodes" where described as docutil nodes
* whether "centered" is for deprecated text or whether it is deprecated
* what is pending_xref_condition
* what means "forcedly"
* Why some script can take no filename while it is marked as a str argument
(by consistency, I changed similarly css)
* why ProductionList counted the number of rules
* how come verify_needs_extensions actually verifies

Co-authored-by: Jakob Lykke Andersen <jakobandersen@users.noreply.github.com>

Co-authored-by: Takeshi KOMIYA <i.tkomiya@gmail.com>
Co-authored-by: Jakob Lykke Andersen <jakobandersen@users.noreply.github.com>

# Conflicts:
#	sphinx/addnodes.py
#	sphinx/application.py